The present invention relates to the field of integrated circuits, and, more particularly, to an integrated circuit and method having the capability of sensing activity.
Over the years, various microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) have arisen which require the necessity to sense temperature, pressure, strain, acceleration, rotation, infrared radiation, chemical properties of liquids and gases, and other physical inputs. Accordingly, various types of microsensors have been developed which receive analog and digital electrical inputs and also sense or measure these other physical inputs, e.g., acceleration, pressure, temperature, strain.
Integrated circuits are widely used in many of these MEMS or electronic applications. Various integrated circuit manufacturing processes, e.g., very large scale integrated (xe2x80x9cVLSIxe2x80x9d) are also widely known and provide various advantages. The complimentary metal oxide semiconductor (xe2x80x9cCMOSxe2x80x9d) manufacturing technology, for example, generally provides a low power dissipation advantage over known metal oxide semiconductor (xe2x80x9cMOSxe2x80x9d) processes. Microsensor manufacturing which is compatible with known integrated circuit manufacturing processes, however, can be quite complicated, especially because of a need for integrating various types of structures at relatively low cost.
Examples of applications for microsensors for acceleration or accelerometers include air bag systems, anti-lock braking systems, and ride suspension systems for automobiles and in-flight aircraft monitoring systems for aircraft. Each of these applications requires small, inexpensive, and reliable acceleration devices.
Many of the known accelerometers for these applications, for example, are analog and measure or sense an electrical current that varies with frequency or amplitude of acceleration. In other words, in essence, many of these sensors convert mechanical parameters to other energy domains and then sense or measure directly. For sensors using direct sensing, the parameters are conventionally related to strain, stress, or displacement. The principles conventionally used to measure or sense strain are piezoelectricity, piezoresistivity, and capacitive or inductive impedance.
The measurement of piezoelectric effects, however, often requires a high input impedance amplifier to measure the surface charges or voltages generated by the stress or the strain. These types of sensors can be expensive and are often not readily acceptable for high density integrated circuit technology and various integrated circuit manufacturing technology.
The measurement of piezoresistivity in conductors and semiconductors conventionally involves the strain on the crystal structure deforming the energy band structure and, thus, changing the mobility and carrier density that changes the resistivity or orientation. These type of sensors, however, are also like piezoelectric sensors in that these sensors can be expensive to manufacture and often may not be very stable for acceleration applications.
Capacitive or inductive impedances can also be used to measure acceleration. Examples of such sensors can be seen in U.S. Pat. No. 5,417,312 by Tsuchitani et al. titled xe2x80x9cSemiconductor Acceleration Sensor and Vehicle Control System Using The Same,xe2x80x9d U.S. Pat. No. 5,506,454 by Hanzawa et al. titled xe2x80x9cSystem And Method For Diagnosing Characteristics Of Acceleration Sensor,xe2x80x9d U.S. Pat. No. 5,610,335 by Shaw et al. titled xe2x80x9cMicroelectromechanical Lateral Accelerometer,xe2x80x9d and U.S. Pat. No. 5,659,195 by Kaiser et al. titled xe2x80x9cCMOS Integrated Microsensor With A Precision Measurement Circuit.xe2x80x9d Capacitive devices integrate the change of elementary capacitive areas while piezoresistive devices take the difference of the resistance changes of bridge arms. Accordingly, capacitive sensors are generally less sensitive to the sideways or indirect forces and are generally more stable. Capacitive sensors, however, conventionally require a capacitance-to-voltage converter on or near the chip to avoid the effects of stray capacitances which can complicate the associated circuitry. The measurement circuitry for these types of sensors is also required to be stable and have low noise.
Additionally, some accelerometers provide a digital output by using a xe2x80x9cspringxe2x80x9d that either makes or breaks an electrical contact in response to acceleration. Some of these spring elements, for example, may provide a series of sensing elements having incrementally higher response thresholds which make electrical contact when the threshold is reached. These xe2x80x9cspringxe2x80x9d accelerometers, however, are relatively large in size as compared to VLSI circuitry and can be quite difficult to make compatible with current integrated circuit manufacturing processes.
Yet further types of microsensors which provide a digital output for detecting translational or rotational acceleration are also known. An example of such a microsensor can be seen in U.S. Pat. No. 5,610,337 by Nelson titled xe2x80x9cMethod of Measuring The Amplitude And Frequency Of An Acceleration.xe2x80x9d One type of accelerometer uses a sensing element which has some sort of pivotally mounted tilting beam (see FIG. 1B therein). The pivotally mounted tilting beam includes a hinge portion, a rigid connection member connected to the hinge portion, and a pair of respective end portions, e.g., a proof mass, connected to the rigid connection member. The end portions rotate clockwise and counter clockwise during rotational, e.g., horizontal, movement. Only one of the end portions contacts a contact electrode which responsively stores the contact signal to indicate that the movement was in the one direction. In essence, this sensing element provides three-states, namely tilted contact in one direction, tilted contact in the other direction, or untilted or neutral. Such a sensing element, however, requires a reset pulse or a reset position which needs to be activated by an external reset: activation source.
Another type of accelerometer which provides a digital output is also illustrated in this patent (see FIGS. 2-4). This sensing element provides a cantilever beam type arrangement that includes a thick beam, a thinner portion of flexible material connected to and extending outwardly from the thick beam and defining a hinge, and a thicker proof mass or end portion connected to and extending outwardly from the hinge. This arrangement of a cantilever beam has problems with xe2x80x9cstuck onxe2x80x9d conditions which also require complex reset structures and conditions. This arrangement also may include small critical dimension which can make manufacturing of such a device difficult and expensive with known integrated circuit manufacturing processes such as CMOS technology.
With the foregoing in mind, the present invention advantageously provides an integrated CMOS sensor and associated methods for sensing acceleration. The present invention also advantageously provides an integrated sensor that is readily compatible with existing integrated circuit manufacturing technology and manufacturing processes, that has greater tolerance for small critical dimensions, and that provides better signal indication when interfacing with logic of an integrated circuit. The present invention additionally provides a cost effective method of forming an integrated sensor for sensing activity desired to be sensed, such as acceleration in a predetermined direction. The present invention further advantageously provides an integrated sensor and associated methods which more reliably and more precisely sense an acceleration event.
More particularly, the present invention advantageously provides an integrated sensor for sensing acceleration in a predetermined direction. The integrated sensor preferably includes a switch detecting circuit region and a sensor switching region connected to and positioned adjacent the switch detecting circuit region. The switch detecting circuit region is preferably provided by a CMOS switch detecting circuit region, such as an inverter circuit. The sensor switching region preferably includes a plurality of floating contacts positioned adjacent and lengthwise extending outwardly from the switch detecting circuit region for defining a plurality of released beams so that each of the plurality of released beams displaces in a predetermined direction responsive to acceleration. The plurality of released beams preferably include at least two released beams lengthwise extending outwardly from the switch detecting circuit region to different predetermined lengths and at least two released beams lengthwise extending outwardly from the switch detecting circuit region to substantially the same predetermined lengths.
According to other aspects of the present invention, the sensor switching region can also include at least one fixed contact layer underlying the plurality of released beams. Preferably, each of the plurality of released beams contacts the at least one fixed contact layer responsive to acceleration in a predetermined direction so as to form a closed switch position. In turn, the switch detecting circuit region generates a signal responsive to the contact of the plurality of released beams with the at least one fixed contact layer.
Additionally, advantageously, the sensor switching region can also include remaining portions of a sacrificial layer positioned between the at least one fixed contact layer and each of the plurality of released beams. Each of the plurality of released beam is positioned in spaced relation from the at least one fixed contact layer in a normally open position. The spaced relation preferably forms by removal of unwanted portions of the sacrificial layer. Each of the plurality of released cantilever beams contacts the at least one fixed contact layer responsive to acceleration in a predetermined direction to form a closed switch position.
Also, at least one insulating layer preferably underlies the at least one fixed contact layer, and the at least one fixed contact layer is preferably a plurality of fixed contact layers. A plurality of insulating layers are also preferably positioned between an adjacent pair of the plurality of fixed contact layers for insulating each of the plurality of fixed contact layers from another one of the plurality of fixed contact layers. The remaining portions of the sacrificial layer preferably include at least one concave surface underlying each of the plurality of released beams. This concave surface, for example, can advantageously be formed by an isotropic etch to release each of the plurality of beams. The plurality of released beams can be either released cantilever beams, released beams each having a configuration including at least two supports, or a combination of these or other types of released beams.
Further, the integrated sensor can advantageously include acceleration calibrating means associated with the plurality of released beams for providing a plurality of calibrated accelerations sensed by the integrated sensor. The acceleration calibrating means preferably includes a predetermined length of each of the plurality of released beams so as to substantially correspond to a plurality of selected calibration threshold regions. Each of the plurality of selected acceleration calibration threshold regions is preferably provided by a portion of the at least one fixed contact layer substantially corresponding to a region of contact of at least one of the plurality of released beams with the at least one fixed plate.
The present invention also advantageously provides methods of forming an integrated sensor. A method of forming an integrated sensor preferably includes providing a switch detecting circuit region and forming a plurality of sensor switching regions connected to and positioned adjacent the switch detecting circuit region. Each of the sensor switching region is preferably formed by at least forming a first conducting layer of material on a support so as to define a fixed contact layer and forming a second floating contact overlying the first conducting layer so as to define a released beam.
According to one aspect of the method, the released beam preferably forms a released cantilever beam. This method, for example, can include depositing a sacrificial layer on the first conducting layer, depositing a second conducting layer on the sacrificial layer, and removing at least unwanted portions of the sacrificial layer, e.g., by etching, to release the second conducting layer so as to define the released cantilever beam.
According to another aspect of the method, the released beam preferably forms a released beam overlying the fixed contact layer so as to have a configuration including at least two supports. This method, for example, can include depositing a sacrificial layer on the first conducting layer, depositing a second conducting layer on the sacrificial layer, and removing at least unwanted portions of the sacrificial layer, e.g., by etching, to release the second conducting layer so as to define the released beam having the at least two supports.
According to other aspects of the method of forming an integrated sensor, the method can further include forming an insulating layer on the support prior to the step of forming the first conducting layer. The fixed contact layer is preferably formed of at least one of polysilicon and a metal, and the second conducting layer is also preferably formed of at least one of polysilicon and a metal.
A method of sensing an activity is also provided according to the present invention. The method preferably includes providing a switch detecting circuit region and providing a plurality of floating contacts positioned in spaced relation in a normally open position with at least one fixed contact layer. Each of the plurality of floating contacts preferably have substantially the same length. The method also includes contacting less than all of the plurality of floating contacts with the at least one fixed contact layer so as to form a closed switch position and generating an activity confirmation signal responsive to a majority of the plurality of floating contacts contacting the at least one fixed contact layer.
This method can also include the plurality of floating contacts being a first plurality of floating contacts, and the method further including providing a second plurality of floating contacts having a different length than the first plurality of contacts, contacting less than all of the second plurality of floating contacts with the at least one fixed contact layer so as to form a closed switch position, and generating an activity confirmation signal responsive to a majority of the second plurality of floating contacts contacting the at least one fixed contact layer.
Therefore, the present invention advantageously provides an integrated sensor and associated methods having a small chip area which allows arrays of sensors to be fabricated on the same die. The present invention also advantageously provides integrated sensors and methods which increase the reliability of the sensing of the activity, such as acceleration or deceleration. The fixed contact layer and the floating contact of the integrated sensor thereby advantageously provide a micro-mechanical sensing region that can readily be formed with known integrated circuit manufacturing processes.